Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large scale manufacture of integrated circuits. A typical ion implanter comprises three sections or subsystems: (i) a terminal for outputting an ion beam, (ii) a beamline for directing and conditioning the beam output by the terminal, and (iii) an end station which contains a semiconductor wafer to be implanted by the conditioned ion beam. The terminal includes a source from which a beam of positively charged ions is extracted. The beamline components adjust the energy level and focus of the extracted positively charged ion beam on its way toward the wafer to be implanted.
A problem encountered in the use of such an ion implanter is that of wafer charging. As the positively charged ion beam continues to impact the wafer, the surface of the wafer may accumulate an undesirable excessive positive charge. Resulting electric fields at the wafer surface can damage microcircuitry on the wafer. The problem of accumulated surface charge becomes more pronounced as implanted circuit elements become smaller, because smaller circuit elements are more susceptible to damage caused by the resultant electric fields.
Another problem encountered in the use of such an ion implanter, especially in low energy applications, is a phenomenon referred to as beam "blow-up", which concerns the tendency for like (positively)-charged ions within the beam to mutually repel each other (also known as the space charge effect). Such mutual repulsion causes a beam of otherwise desired shape to diverge away from an intended beamline path. Beam blow-up is particularly problematic in high current, low energy applications because the high density of ions in the beam (high current) exaggerates the force of mutual repulsion of the ions, and the small velocities (low energy) of the ions allows more time for the repulsive force to act upon the ions before they reach the wafer.
A known solution to both wafer charging and the beam blow-up phenomenon is the use of an electron or plasma shower. Such showers may also be referred to as electron or plasma floods. Both electron and plasma showers generate low energy electrons and introduce these electrons into the beam. Plasma floods generate a plasma in an arc chamber and the ion beam potential extracts low energy plasma and electrons into the beam. Electron showers generate secondary (low energy) electrons which are used to enhance the beam to reduce space charge (beam blow-up) tendencies and wafer charging effects.
A typical electron shower shower includes a target chamber in which secondary electrons are generated and an extension tube connected downstream of the target chamber. As the ion beam passes through the target chamber, secondary electrons infiltrate and partially neutralize the beam. The partially neutralized beam passes through the extension tube toward the wafer to be implanted. The trapped low energy electrons thereby neutralize the net charge of the beam which in turn reduces the positive charge accumulation on wafer as the ion beam strikes the wafer surface. The neutralized beam is also less likely to experience detrimental beam blow-up characteristics. Such a system is shown in U.S. Pat. No. 4,804,837 to Farley, assigned to the assignee of the present invention and incorporated by reference as if fully set forth herein.
The extension tube of a typical electron shower is made of graphite which provides a conductive path to ground for high energy primary electrons which are preferably prevented from reaching the surface of the wafer. High energy primary electrons could otherwise negatively charge and damage the surface of the wafer. Low energy secondary electrons are preferably passed through the extension tube along with the ion beam to beneficially neutralize positive wafer charging, without negatively charging the wafer.
When the graphite extension tube is new, however, it provides a highly conductive shunt path to ground. As such, even secondary low energy electrons are shunted to ground, removing these shunted electrons from the available supply of charge neutralizing secondary electrons. Over the course of operation of the electron shower, the ion beam passing therethrough impacts the wafer to be implanted, causing a sputtering effect of surface material such as photoresist or silicon or silicon dioxide. The photoresist or other material is back-sputtered to the interior surface of the extension tube, which is the closest portion of the electron shower to the wafer.
Soon after a thin layer of photoresist or other material is back-sputtered to the extension tube, optimal operation results. Because the tube has become slightly less conductive, the tube will charge negatively from the impinging electrons and create a potential barrier preventing low energy electrons from being shunted to ground. Thus, low energy secondary electrons are passed through the tube along with the ion beam to serve their charge neutralizing function. Primary high energy electrons, however, continue to be shunted to ground. Thus, an extension tube must be in use for a "break-in" time before it operates as intended.
As operation continues, however, effectiveness of the extension tube deteriorates. As back-sputtering of photoresist or other material continues, the extension tube becomes increasingly electrically insulating. If the tube becomes sufficiently insulating, even high energy primary electrons will be undesirably passed on to the wafer. In addition, high energy primary electrons which strike the inner insulating surface of the extension tube charge this surface. Secondary electrons which may originate on the surface of the tube, rather than the chamber, therefore assume energy levels consistent with the potential of the insulating surface. The energy level and current density of the secondary electrons therefore drifts higher and higher as the photoresist or other material coating continues to thicken over time, creating more undesirable high energy secondary electrons.
The effectiveness of a particular electron shower depends in part on the efficient generation of a sufficient supply of secondary electrons having consistently low and predictable energy levels. Accordingly, it is an object of the present invention to provide an electron or plasma shower extension tube which increases the number of secondary electrons which are available for charge neutralization purposes, while insuring that secondary electron energy levels are minimal and constant, by providing better control over the energy levels of secondary electrons passed therethrough.
It is a further object of the present invention to provide an electron or plasma shower extension tube which prevents high energy primary electrons from reaching the surface of the wafer.
It is yet a further object of the present invention to provide an electron or plasma shower extension tube which minimizes the adverse effects of back-sputtered contamination.